This invention relates to the field measurement of mineral, plant, and general surface spectral radiance, and more particularly, to an improved reflectance standard for such measurements.
Radiometers are used in geological exploration to detect outcrops and for geological mapping. Radiometer measurements are used to classify soil, overburden and rocks, and they are used in stratigraphical studies and examination of lithological changes. A typical radiometer used in such studies is the Barringer Research, Ltd. ratioing radiometer described in that company's brochure entitled "BARRINGER RESEARCH RATIOING RADIOMETER, HAND-HELD REFLECTANCE RADIOMETER FOR THE EARTH SCIENCES". In geological exploration, the radiometer is used to detect radiant energy reflected from a target such as the rock being investigated. The detected energy is converted to an electrical signal which is compared to a signal produced by detecting radiant energy from a standard.
Accurate reflectance standards are critical in these investigations. The current practice is to use a material consisting of finely spun filaments of glass in wooly masses of the type commonly used for insulation.
More specifically, the Fiberfrax ceramic fiber felt, made by the Carborundum Company, have been extensively used as a reflectance standard. Ihe use of this material is described in "Optical Characteristics of a Proposed Reflectance Standard" Trytten and Flowers, Applied Optics, Dec. 1965, Vol. 5, No. 12, and in "Spectral and Bi-Directional Reflectance of Pressed Versus Unpressed Fiberfrax", Applied Optics, Vol. 10, No. 7, July 1971.
One of the desirable characteristics of a reflectance standard is that it have a Lambertian surface. That is, the reflectance should be the same regardless of the orientation of the standard with respect to the source of radiation.
Fiberfrax ceramic fiber felt has very good optical characteristics for use as a standard, but it has drawbacks, particularly for field use. This felt does not have a Lambertian surface. For this reason, in field use, Fiberfrax ceramic fiber felt standards must be used with the same orientation between the source of radiation and the standard, and between the standard and the radiometer. Also, it has a crumbling surface which is particularly susceptible to contamination by dirt. The present practice is to periodically scrape off the top layer containing the dirt so that a fresh surface is always presented for reflectance measurements. Because of this, a ceramic fiber felt reflectance standard is commonly used for only a day of field measurements.
Attempts to replace ceramic fiber felt as a reflectance standard for field use have included test cards having surfaces of standard reflectance as described, for example, in "Field Standards of Reflectance", Palmer, Photogrametric Engineering and Remote Sensing, Vol. 48, No. 10, Oct. 1982, pp. 1623-1625. These cards do not have a Lambertian surface and they do not have a high efficiency spectral response of the type required for measurements made in geological exploration. For these reasons the cards have not been extensively used in geological exploration.
It is an object of the present invention to provide an improved reflectance standard particularly suitable for field use in geological studies.
It is another object of the present invention to provide a reflectance standard having a Lambertian surface and a surface which is relatively free from dirt contamination.